The present invention relates generally to measuring reactive power.
Some electrical equipment, such as computers and air conditioners, may add non-linear loads to power lines. Thus energy distributors need to measure reactive power, in addition to active power, to bill the total energy delivered. Active power and reactive power are respectively defined as:Active power=U*I*Cos(θ), and  (1)Reactive power=U*I*Sin(θ),  (2)where U and I are the voltage and current RMS (Root Mean Square) values respectively, and θ is the phase difference between the voltage and the current.
Equations (1) and (2) indicate that the active power and reactive power have a 90 degree phase offset, so one approach to measure reactive power is to add a 90 degree phase shift to the active power. FIG. 1 illustrates a prior art system for measuring active power. The voltage and current are multiplied at a mixer 101 and the result is filtered by a low pass filter (LPF) 102. FIG. 2 illustrates a prior art system for measuring reactive power, with a 90 degree phase shift added to the current path. The phase shifted current may be multiplied with the voltage at a multiplier 201 and the result may be filtered by a LPF 202. Alternatively, the 90 degree phase shift may be added to the voltage path.
Prior art systems use a time delay, a low-pass filter or a Hilbert transform filter to implement the 90 degree phase shifting in a power meter. The time delay method may add a T/4 delay to shift a waveform by 90 degrees, where T is the period of the waveform. Although the time delay method is simple, significant errors may be introduced to the result. Low-pass filters may add a constant 90 degree phase shift over frequency, but they are not good enough with harmonics. A Hilbert transform filter may introduce a 90 degree phase shift over any frequency including fundamental and harmonics, but it requires a dedicated process to get a constant phase shift of 90 degrees at each frequency, and maintaining a flat gain response at each frequency is not easy.
Therefore, it is desirable to provide a system and method for measuring reactive power in a simple but accurate way.